Hezbollah’s Katyusha rockets are used by them to taunt and poke Israel. They’re WWII era, Soviet made, relatively easy to defend.
They keep Israel and the IDF off balance.
They prevent the return of 80,000 Israelis to their homes in the North.
They want to keep the entire country of Israel on edge, disrupt business and tourism.
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The idea that the IDF hit Lebanon as a preemptive strike is incorrect.
This was offensive in nature, and simply ended with the Hezbollah response described above.